Inspect own function properties

See #67.

We might want to consider if the amount of output should be limited for functions that have a lot of properties, or where the dumps of those properties take up a lot of space.

Presently this is missing cycle detection:

```js
function foo() {}
foo.bar = foo;
expect(foo, 'to have property', 'baz');
// RangeError: Maximum call stack size exceeded
```

When solving that, another possible refinement could be to render the cyclic reference as the name of the function.